Opposition leader Sergei Udaltsov gestures as he boards a police van after being detained during a protest march in Moscow on Saturday. Russian activists fear that Mr. Udaltsov will soon be the subject of a show trial, and draw comparisons to such trials under Stalin in the 1930s.

"We are definitely fearful that authorities are preparing a mega-trial," perhaps based on the alleged riot that took place during a mostly peaceful street protest on May 6, the day before Mr. Putin's inauguration for a third term, says Yevgeny Ikhlov, information officer with For Human Rights, a grass-roots Moscow-based coalition.

"I know it sounds mad. Nobody will believe in this big conspiracy [that the Kremlin is alleging], and the authorities' credibility will suffer, but we see all the signs that it's being worked up," he says.

According to the script being prepared by the Kremlin, the activists allege, left-wing leader Sergei Udaltsov will be the principal defendant. He was accused in a "documentary" film, broadcast on the state-run NTV network, of plotting violent revolution at the behest of an official of the Georgian government and bankrolled by exiled anti-Putin tycoons in London.

Mr. Udaltsov was charged last Friday with conspiracy to conduct "mass disorders," which carries a potential 10-year prison sentence. Also charged were his associates, Konstantin Lebedev and Leonid Razvozzhayev, who alleges he was kidnapped in Ukraine last week by Russian secret services, illegally transported to a Russian prison, and forced to "confess" under torture.

The Russian official narrative about Mr. Razvozzhayev has unraveled in recent days. According to Investigative Committee spokesman Vladimir Markin, he voluntarily gave himself up and penned a 10-page confession about his role in the vast anti-Kremlin conspiracy.

But, under pressure from international governments and groups like Amnesty International, Ukraine's security service, the SBU, posted a terse statement on its official website last Friday insisting that it had nothing to do with the case, and that Razvozzhayev had been abducted by "the security forces of another country" and transferred "in a hurried manner" to Russia.

When finally given access to his lawyer and members of the Public Monitoring Commission, an officially sanctioned prison watchdog, Razvozzhayev repudiated his confession and said he had been coerced into making it with threats made against his family and physical pressures such as sleep and food deprivation.

"There is no doubt that [Razvozzhayev] was tortured," former judge and prisoners' rights activist Valery Borshchyov told journalists.

"It is usually thought that torture has to involve beating, blood, and so on, but it is not true. There are other kinds of torture. He told us how he was kidnapped and put in a van, how [the abductors] tied his feet and hands with tape and put a hat on his face so he couldn't see anything...."

"Our country has entered a new era of Stalin-like repressions," Mr. Borshchyov added.

References to the 1930s Stalin-era show trials, which smashed any semblance of opposition to Stalin and cowed Soviet society for generations, have been increasingly invoked by worried human rights activists and opposition leaders since last summer's Pussy Riot trial raised fears of an arbitrary state using criminal sanctions to intimidate political opponents.

But the parallel is deeply controversial. Many observers reject loose analogies with the Stalin era as wildly exaggerated, arguing correctly that it diminishes the extremes of Stalinism, and misrepresents the realities of Putin's Russia, which, though increasingly authoritarian, cannot be usefully compared to the state absolutism and mass terror that characterized the USSR in the 1930s.

Nevertheless, many people whose memories of the Stalin period remain fresh are deploying that comparison today, arguing that they see similarities in the state's actions with respect to Pussy Riot and the looming Udaltsov case that suggest a worrisome continuity.

And it's not only the liberal intelligentsia invoking such historical sensitivities to dramatize their points. Many religious conservatives who defended the harsh sentences meted out to the Pussy Riot women – and not only in Russia – argued in a similar vein that the feelings of Russian Orthodox believers, traumatized by Soviet-era persecutions, were deserving of special protection.

"A lot of things going on today remind us of Stalin times, even if most things are very different," says Yury Schmidt, one of Russia's leading defense lawyers.

"Society has changed a lot, and I do not believe that people can be frightened to death the way they were then, and yet our authorities do seem to be preparing to hold this trial, though I am not at all sure they can find the actors and manage the process successfully.... I was born in 1937, have seen such things in my life that I've lost all fear. After I defended [disgraced oil tycoon Mikhail] Khodorkovsky, I thought the authorities had lost interest in me, but lately I begin to feel that I am in the focus of someone's attention again," he adds in an ominous tone.

Sergei Strokan, a columnist with the daily Kommersant, says a key reason Stalin keeps cropping up in Russian discourse is that the country never underwent a thorough process of de-Stalinization; hence the fear of its revival is ever present.

"Of course it's unreasonable to see the ghost of Stalin inhabiting Putin's body, and even more so to see the spirit of Stalinist repression against the church parading in the form of Pussy Riot," he says.

"But it's not amiss to note that the state still has supreme power in this country, and it still claims absolute priority over the lives of individuals. This is the quintessential idea of the Soviet state, and it lives on today. It is that the state is equivalent to the nation; hence, if you attack Putin, you're attacking Russia.... We do see the creeping revival of that old system, and it's something to really worry about," Mr. Strokan adds.

Vladimir Pribylovsky, director of the Panorama Center, an independent political consultancy, says the public atmosphere in Russia is already dramatically changed from a year ago, and a big show trial may deeply intimidate society.

"We have several dozen people who can be described as political prisoners, and it looks like some big trials are being prepared," he says.

"People are going to be afraid to talk to foreigners, just as in the past, and among left-wing activists we already see paranoiac moods taking shape.... This is a long-term trend, and it's going to impact our society in unpredictable ways," he adds.